This invention relates to the handling of cast concrete slabs, and more particularly, to novel development in connection with the use of a coil insert or nut member for embedding in a cast concrete slab or panel to receive a bolt used to attach detachable hoisting means for lifting the slab.
In recent years, a form of building construction referred to as the tilt-up slab method has come into rather wide usage. In tilt-up slab building, large concrete slabs are formed either on the floor of the building or on a level ground surface. The concrete slabs are then lifted to a vertical position and interconnected to form the walls of the building.
The attachments to the slabs which are used for lifting them must be removable after the slabs have been lifted into place to facilitate the provision of a smooth surface on the walls of the building. Generally, the lifting attachments, such as a lifting ring and bolt, are installed and removed at the building site, and therefore they are designed to be easily and rapidly installed and removed in order to save maximum time.
The insert into which the bolt is threaded is normally permanently embedded in the slab or panel. When the lifting attachments are removed from the slab, a hole where the insert is located remains in the slab and must be filled to provide a smooth outer surface. The insert should therefore be small enough to be easily and readily concealed after the slab has been installed, and yet should be able to sustain the forces of lifting heavy slabs or panels. Also, it should not interfere with rapid attaching and detaching of the lifting means.
The art has for many years used bolts of conventional types having relatively course threads, including nut headed bolts for use with a lifting device in which a hoist can be attached, and also eye bolts which do not require separate lifting rings. A more recent development in the art is a split bolt such as shown in Strong U.S. Pat. No. 3,456,547 or Grayson U.S.Pat. No. 3,922,946 which can be rapidly inserted without threaded into an insert in a slab and then tightened in place by only a partial revolution.
Such a split bolt comprises a pair of separate longitudinal segments such as would be formed if a conventional bolt is split along its axis and then has a portion removed along the axis from each segment so that when the two segments are placed together, they can be inserted axially into an insert sized for threaded reception of the entire bolt. An axially movable key member or wedge is used to replace the removed portions and thereby to spread the two segments into threaded engagement with the insert, and a partial turn of the bolt will then tighten it securely in place. The bolt is removed by withdrawing the key, thereby enabling the two segments to be brought together out of threaded engagement with the insert for axial withdrawal without rotation.
The inserts commonly used with both solid bolts and split bolts comprise a coil of a plurality of turns of heavy wire or rod stock welded to supporting leg members or the like for locating the coil in proper position in a poured concrete slab or panel, commonly by setting the foot portions of the leg members on the form on which the slab is poured. Experience has established that there is a practical minimum number of turns in a coil and threads on a coil bolt which must be in engagement to assure adequate lifting strength without causing the coil turns to separate and to release the bolt.
A number of alternative means have been used in the art for this space-establishing purpose, including plugs of various kinds which serve to prevent access of fluid concrete both to the interior of the coil and into a space immediately adjacent the inner end of the coil, and which can be burnt out or otherwise removed after the concrete is set. Among patents showing a plug of special characteristics for this purpose are Boll et al. U.S. Pat. No. 2,880,608, Jenkins U.S. Pat. No. 3,216,171, Holt U.S. Pat. No. 3,590,538 and Tye U.S. Pat. No. 3,742,661. The plugs of all of such patents, however, offer certain disadvantages from the standpoint of both cost and simplicity of use, as now briefly summarized.
Boll et al. discloses the use of a filler plug described as composed of sponge rubber or some easily compressed rubber or elastic material allowing the plug to be easily inserted and to retain itself in place yet to be easily subsequently removable. Boll, however, does not teach how to insert or remove the plug, and apparently it would have to be crammed into place and subsequently dug out piecemeal. Each of the other three patents shows a two-part plug which must be separately threaded or otherwise fitted in place at opposite ends of the coil prior to installation, with the outer plug being removed after the concrete has set but with the inner plug remaining in place for partial destruction by the coil bolt as it is threaded into the insert.
In U.S. Pat. No. 3,889,916, issued June 17, 1975 to Ilukowicz, a solid plastic protective plug for an insert is disclosed. This insert is configured only for use with edge inserts which are positioned adjacent a mold wall and, additionally, must be threaded completely into and out of the coil. Insertion and removal of the insert is therefore time consuming.